You need to reformat the PC drive for a Mac to use the drive for a Time Machine backup.

Type Disk Utility (into Spotlight).

Erase

Select OS X Extended Journaled

Under options select GUID

then proceed to format the drive
(be warned: anything that is on this drive will be erased)

Then under time machine preferences you can designate that drive as your backup.

p.s. you may be able to do this formatting through time machine preferences as indicated below, but I've never done this.

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From Apple:

Note: Every available drive that can be used to store backups is listed. If youíve partitioned a drive, the available partitions are listed. Time Machine canít backup to an external drive that's connected to an AirPort Extreme, Time Capsule, or a drive formatted for Microsoft Windows (NTFS or FAT format). If you select an NTFS or FAT-formatted drive, Time Machine prompts you to reformat the drive. Choose a different drive or reformat the drive in Mac OS Extended (Journaled) format. Because reformatting erases any files on the drive, only do this if you no longer need the files or if you have copies of them on a different drive.

The most common format for a Time Machine backup drive is Mac OS Extended (Journaled) format, but Time Machine also supports Mac OS Extended (Case sensitive, Journaled) and XSan formats.

If the drive is partitioned using the Master Boot Record (MBR) partition type, some partitions may not be available for use with Time Machine. The GUID Partition Table (GPT) or Apple Partition Map (APM) partition types are recommended.

Joined: 19 Aug 2006Posts: 724Location: A large Island called Vancouver

Posted: Fri Jul 06, 2012 2:00 pm Post subject:

Your first post mentions an Airport "Extreme" which I have and use the external disk for TM.
Your second post refers to the Airport Express, which I've had no experience with and cannot comment on.
Are you comparing Apples & Oranges here?_________________"I would never want to be a member of a group whose symbol was a man nailed to two pieces of wood"
-George Carlin

Joined: 19 Aug 2006Posts: 724Location: A large Island called Vancouver

Posted: Fri Jul 06, 2012 2:10 pm Post subject:

Just took a look at the Apple support and I can see where you're coming from.
I am using a first generation Airport extreme and originally took advantage of the Apple update mentioned in the article I linked to.
Makes me glad I'm not in the habit of jumping on all of the updates Apple pushes out, or I might just have broke a good working system._________________"I would never want to be a member of a group whose symbol was a man nailed to two pieces of wood"
-George Carlin

Joined: 19 Aug 2006Posts: 724Location: A large Island called Vancouver

Posted: Fri Jul 06, 2012 6:43 pm Post subject:

You're right, it does suck, but I'm not surprised.
It amazes me how quickly Apple is becoming the dreaded "big brother" it was rallying against in the now famous "1984" commercial._________________"I would never want to be a member of a group whose symbol was a man nailed to two pieces of wood"
-George Carlin